A civilian used the officer's radio to notify police about the shooting, she said.

Bolton, who joined the force in October 2010, was taken to hospital in critical condition. He later died.

This is the third time a Memphis police officer has been killed in the past four years, Memphis Police Director Toney Armstrong said.

\"It doesn't get any easier. This is a very difficult time, not only for me as director, but for all the officers you see standing behind me,\" he told reporters early Sunday morning.

\"And (it's) a difficult time for our city as well. So I ask that you give us the respect that we need ... and the space that we need to grieve, and the time that we need to prepare to lay our brother to rest.\"

The shooting follows a series of incidents across the country in which police have been accused of excessive force at traffic stops.

Former University of Cincinnati Police Officer Ray Tensing faces charges of murder and voluntary manslaughter for shooting Samuel DuBose in the head during a traffic stop July 19. DuBose's death reignited a familiar chant across the country: \"Black lives matter.\"

It's unclear what led to Bolton's shooting, but the police chief highlighted the risks police officers face on the job.

\"We say so often ... do black lives matter? And at the end of the day, we have to ask ourselves, do all lives matter -- regardless of race, creed, color, economic status, what profession that person holds? All lives matter,\" Armstrong said.